Cascaded organic rankine cycle (orc) system using waste heat from a reciprocating engine

ABSTRACT

A method and system for operating a cascaded organic Rankine cycle (ORC) system ( 100 ) utilizes two waste heat sources from a positive-displacement engine ( 106 ), resulting in increased efficiency of the engine ( 106 ) and the cascaded ORC system ( 100 ). A high temperature waste heat source from the positive-displacement engine ( 106 ) is used in a first ORC system ( 102 ) to vaporize a first working fluid ( 118 ). A low temperature waste heat source from the positive-displacement engine ( 106 ) is used in a second ORC system ( 104 ) to heat a second working fluid ( 130 ) to a temperature less than the vaporization temperature. The second working fluid ( 130 ) is then vaporized using heat from the first working fluid ( 118 ). In an exemplary embodiment, the positive-displacement engine ( 106 ) is a reciprocating engine. The high temperature waste heat source may be exhaust gas and the low temperature waste heat source may be jacket cooling water.

BACKGROUND

The present disclosure relates to an organic Rankine cycle (ORC) system. More particularly, the present disclosure relates to operating a cascaded ORC system using two waste heat sources from a reciprocating engine.

Rankine cycle systems are commonly used for generating electrical power. The Rankine cycle system includes an evaporator or a boiler for evaporation of a motive fluid, a turbine that receives the vapor from the evaporator to drive a generator, a condenser for condensing the vapor, and a pump or other means for recycling the condensed fluid to the evaporator. The motive fluid in Rankine cycle systems is often water, and the turbine is thus driven by steam. An organic Rankine cycle (ORC) system operates similarly to a traditional Rankine cycle, except that an ORC system uses an organic fluid, instead of water, as the motive fluid.

The ORC system uses a waste heat source to provide heat to vaporize the organic fluid in the evaporator. A reciprocating engine is a common source of waste heat for an ORC system. Usable waste heat from the reciprocating engine may include exhaust gas at temperatures near approximately 540 degrees Celsius (approximately 1000 degrees Fahrenheit), as well as cooling water at approximately 105 degrees Celsius (approximately 220 degrees Fahrenheit). Challenges arise in trying to use both of the waste heat sources from the reciprocating engine, particularly given the temperature difference between them. As such, the exhaust gas is typically preferred over the cooling water, given the potential for greater heat transfer.

To effectively utilize the high-temperature exhaust heat from the reciprocating engine, the ORC system typically uses an organic fluid with a high critical temperature, allowing boiling at elevated temperatures. However, expanding an organic fluid with a single turbine over a large pressure ratio causes the vapor exiting the turbine to be more superheated, thus limiting the amount of power captured by the turbine. The highly superheated fluid exiting the turbine may also require special condensation equipment.

There is a need for an improved method and system of recovering waste heat from a reciprocating engine in order to increase efficiency of the reciprocating engine and the ORC system.

SUMMARY

A method and system for operating a cascaded organic Rankine cycle (ORC) system utilizes two waste heat sources from a positive-displacement engine, resulting in increased efficiency of the engine and the cascaded ORC system. A high temperature waste heat source from the positive-displacement engine is used in a first ORC system to vaporize a first working fluid. A low temperature waste heat source from the positive-displacement engine is used in a second ORC system to heat a second working fluid to a temperature less than the vaporization temperature. The second working fluid is then vaporized using heat from the first working fluid. The first working fluid has a higher critical temperature than the second working fluid. In an exemplary embodiment, the positive-displacement engine is a reciprocating engine and the waste heat sources are exhaust gas and jacket cooling water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an organic Rankine cycle (ORC) system designed to produce electrical power using waste heat.

FIG. 2 is a schematic of a cascaded ORC system with a first ORC system and a second ORC system, designed to utilize two waste heat sources from a reciprocating engine.

FIG. 3 is a T-s diagram for the cascaded ORC system of FIG. 2.

DETAILED DESCRIPTION

A waste heat recovery system, such as an organic Rankine cycle (ORC) system, may be used to capture heat from a prime mover, such as a reciprocating engine. The ORC system may then be used to generate electrical power. A reciprocating engine has two sources of waste heat that may be recoverable by the ORC system—exhaust gas (high temperature) and cooling water (low temperature). However, given the large temperature difference between the waste heat sources, it is difficult to effectively utilize both of these waste heat sources in a single ORC system. As described herein, in a cascaded ORC system, a first ORC system utilizes a high temperature working fluid to power a generator and a second ORC system utilizes a low temperature working fluid to power a second generator. The first ORC system recovers heat from the exhaust gas of the reciprocating engine. The second ORC system recovers heat from the cooling water of the reciprocating engine, as well as the heat of condensation from the high temperature working fluid of the first ORC system. The cascaded ORC system and method described herein utilizes more of the waste heat from the reciprocating engine, and thus generates a greater amount of power per unit of waste heat from the reciprocating engine.

FIG. 1 is a schematic of a single ORC system 10, which includes condenser 12, pump 14, evaporator 16, and turbine 18. Working fluid 22 circulates through system 10 and is used to generate electrical power. Liquid working fluid 22 a from condenser 12 passes through pump 14, resulting in an increase in pressure. High pressure liquid fluid 22 a enters evaporator 16, which utilizes heat source 24 to vaporize fluid 22. Heat source 24 may include, but is not limited to, any type of waste heat resource, including reciprocating engines, fuel cells, and microturbines, and other types of heat sources such as solar, geothermal or waste gas. Working fluid 22 exits evaporator 16 as a vapor (22 b), at which point it passes into turbine 18. Vaporized working fluid 22 b is used to drive turbine 18, which in turn powers generator 28 such that generator 28 produces electrical power. Vaporized working fluid 22 b exiting turbine 18 is returned to condenser 12, where it is condensed back to liquid 22 a. Heat sink 30 is used to provide cooling to condenser 12.

In those cases in which heat source 24 is a high temperature heat source, working fluid 22 is preferably a high temperature fluid having a high critical temperature. In that case, heat source 24 is able to transfer sufficient heat to the working fluid, while maintaining the working fluid below the critical temperature in evaporator 16. A disadvantage of such a high temperature working fluid, however, is that when it exits turbine 18, it is highly superheated. At least a portion of the heat from the superheated vapor is not converted into power, and thus turbine 18 has a low efficiency. Moreover, the high temperature working fluid requires additional cooling in condenser 12, resulting in expensive equipment and typically a large amount of unrecoverable waste heat from the working fluid.

In contrast, if heat source 24 is a low temperature heat source, a low temperature working fluid may be used within system 10. However, there is a reduced efficiency in power output, as compared to when system 10 recovers heat from a high temperature heat source.

In the scenario in which heat source 24 is waste heat from a reciprocating engine, ORC system 10 typically uses either the exhaust gas (i.e. high temperature waste heat) or the jacket cooling water (i.e. low temperature waste heat), since it is difficult to use both. As such, some of the waste heat from the reciprocating engine is unrecoverable by ORC system 10.

FIG. 2 is a schematic of cascaded ORC system 100 having first ORC system 102 and second ORC system 104, both of which recover waste heat from reciprocating engine 106. First ORC system 102 is similar to ORC system 10 of FIG. 1 and includes evaporator 110, turbine 112, condenser 114, and pump 116. First working fluid 118 is circulated through system 102 and used to drive turbine 112, which enables generator 120 to produce electrical power. Second ORC system 104 includes turbine 122, condenser 124, pump 126, heat exchanger 128, and evaporator 114. Second working fluid 130 is used in second ORC system 104 to drive turbine 122, which powers generator 132. Condenser 124 of second ORC system 104 uses heat sink 134 to provide cooling and condense vaporized working fluid 130 from turbine 122. Heat sink 134 may be water or air, and in some cases, heat sink 134 may be used to provide useful heating to an external source, as discussed further below. First working fluid 118 and second working fluid 130 are organic working fluids, examples of which are provided below.

Condenser 114 of first ORC system 102 also functions as the evaporator of second ORC system 104. As described further below, first working fluid 118 is a high temperature working fluid and second working fluid 130 is a low temperature working fluid. As such, evaporator/condenser 114 is configured such that vaporized working fluid 118 from turbine 112 is condensed, thereby transferring heat to vaporize second working fluid 130.

Reciprocating engine 106 has two sources of waste heat recoverable by system 100. The first source is exhaust gas ranging in temperature from approximately 475 to 540 degrees Celsius (approximately 885 to 1005 degrees Fahrenheit). The second source is jacket cooling water with a temperature range of approximately 100 to 110 degrees Celsius (approximately 212 to 230 degrees Fahrenheit). Heat from the exhaust gas is used by first ORC system 102. More specifically, exhaust gas is used by evaporator 110 to vaporize working fluid 118.

Second ORC system 104 receives heat from the jacket cooling water. Heat exchanger 128 of system 104 is located between pump 126 and evaporator 114, and is designed to transfer heat from the jacket cooling water to liquid working fluid 130. Because jacket cooling water is a lower temperature waste heat source, as compared to the exhaust gas, the jacket cooling water is used to heat working fluid 130 to a temperature that is less than its vaporization temperature. Thus, working fluid 130 has a higher temperature at an outlet of heat exchanger 128 compared to its temperature at an inlet of heat exchanger 128. The jacket cooling water may be recycled back to reciprocating engine 106 after exiting heat exchanger 128.

After passing through heat exchanger 128, second working fluid 130 passes through condenser/evaporator 114, which is designed to transfer heat between first working fluid 118 and second working fluid 130, such that first working fluid 118 condenses to a liquid and second working fluid 130 is vaporized. First working fluid 118 preferably has a condensation temperature that is suitable to boil second working fluid 130.

Second working fluid 130 passes from evaporator 114 to turbine 122, and then to condenser 124, which may be a water-cooled condenser or an air-cooled condenser (i.e. heat sink 134 is water or air). In some embodiments, after water in heat sink 134 exits condenser 124, the heated water may be used to provide heating to a source external to cascaded ORC system 100. For example, heat sink 134 may be used to heat district heating water and/or provide environmental heating, for example, to agricultural crops or greenhouses.

Using cascaded ORC system 100, it is possible to utilize essentially all of the waste heat from reciprocating engine 106. The high temperature waste heat source (the exhaust gas) is recovered by ORC system 102 which utilizes a high temperature working fluid. The low temperature waste heat source (the jacket cooling water) is recovered by ORC system 104, which utilizes a low temperature working fluid. Moreover, the design of cascaded ORC system 100 results in greater efficiency overall since the heat from first working fluid 118 exiting turbine 112 may be transferred to second working fluid 130. An efficiency of second ORC system 104 is increased by preheating second working fluid 130 in heat exchanger 128. Moreover, the heat utilization efficiency of ORC system 100 may be further increased by using heat sink 134 to heat a source external to cascaded ORC system 100.

First working fluid 118 has a higher critical temperature than second working fluid 130. Because exhaust gas from reciprocating engine 106 is used in evaporator 110 to vaporize first working fluid 118, working fluid 118 preferably has a high critical temperature such that it is able to boil at a high temperature inside evaporator 110. Operating with the working fluid in the supercritical phase presents technical challenges that are preferably avoided by remaining below the critical temperature.

On the other hand, since second ORC system 104 uses lower temperature heat sources (i.e. cooling water and lower-temperature condensation heat of working fluid 118) to vaporize second working fluid 130, working fluid 130 preferably has a low critical temperature compared to working fluid 118. If a working fluid with a high critical temperature were used in second ORC system 104, the pressures inside system 104 may become too low, resulting in low fluid densities and requiring larger equipment.

First working fluid 118 may include, but is not limited to, siloxanes, toluene, isobutene, isopentane, n-pentane and 4-trifluoromethy1-1,1,1,3,5,5,5-heptafluoro-2-pentene ((CF₃)₂CHCF═CHCF₃). Examples of siloxanes that are suitable for first working fluid 118 include, but are not limited to, MM hexamethyldisiloxane (C₆H₁₈OSi₂), MDM octamethyltrisiloxane (C₈H₂₄O₂Si₃), and MD2M decamethyltetrasiloxane (C₁₀H₃₀O₃Si₄). In some embodiments, siloxanes may be preferred over toluene, isobutene, isopentane, and n-pentene, which are flammable.

Second working fluid 130 may include, but is not limited to, R123, R134a, R236fa and R245fa. In preferred embodiments, R134a or R245fa is used in ORC system 104. If an ambient air temperature is cooler, thereby reducing a temperature of heat sink 34, then R134 may be preferred; if the ambient air temperature is warmer, then R245fa may be preferred.

It is recognized that first working fluid 118 and second working fluid 130 may include organic working fluids not listed above. Numerous combinations of first working fluid 118 and second working fluid 130 may be used. As stated above, cascaded ORC system 100 is preferably operated with first working fluid 118 having a higher critical temperature than second working fluid 130.

FIG. 3 is a T-s diagram for cascaded ORC system 100 of FIG. 2. For both first working fluid 118 and second working fluid 130, temperature T is plotted as a function of entropy S. As described in more detail below, FIG. 3 illustrates the thermal energy transfer from the exhaust gas of reciprocating engine 106 to first working fluid 118, and from the jacket cooling water of engine 106 to second working fluid 130. As also shown in FIG. 3, first working fluid 118 transfers heat to second working fluid 130, and second working fluid 130 then transfers heat to heat sink 134.

Heat from the exhaust gas of reciprocating engine 106 is transferred to first working fluid 118, which increases a temperature of working fluid 118 until fluid 118 reaches its vaporization temperature, as shown in FIG. 3. Fluid 118 remains below the critical temperature T_(1 critical). As vaporized fluid 118 expands in turbine 112, its temperature decreases, however fluid 118 remains in the vapor phase. In condenser 114, which also functions as an evaporator for second ORC system 104, fluid 118 is desuperheated until it reaches its condensation temperature. The heat from fluid 118 is transferred to second working fluid 130 in condenser/evaporator 114. The temperature of fluid 130 remains below the critical temperature T_(2 critical).

Heat from first working fluid 118 is sufficient to vaporize second working fluid 130 inside condenser/evaporator 114. This is due, in part, to preheating of second working fluid 130 upstream of condenser/evaporator 114. As shown in FIG. 3, jacket cooling water from reciprocating engine 106 is used to increase a temperature of working fluid 130 to a temperature below the vaporization temperature.

As similarly described for fluid 118, second working fluid 130 shows a decrease in temperature after passing through turbine 122. At that point, superheated fluid 130 is condensed inside condenser/heater 124 using ambient air or cooling water from heat sink 134. In other words, heat from second working fluid 130 is transferred to heat sink 34, as shown in FIG. 3. As described above, heat sink 34, in some embodiments, may be used to provide heating to an external source, such as, for example, a greenhouse.

In the exemplary embodiment of FIG. 2, cascaded ORC system 100 uses two waste heat sources from a reciprocating engine. The low temperature heat source is jacket cooling water. It is recognized that other types of positive-displacement engines, in addition to reciprocating engines, that require cooling water during engine operation may also be used to supply waste heat to system 100. This may include, but is not limited to, rotary engines, such as, for example, the Wankel engine.

The cascaded ORC system described herein uses two distinct waste heat sources from a reciprocating engine. Since two ORC systems are used, the cascaded ORC system generates additional power. Because there is no change in the emission levels of the reciprocating engine, the cascaded ORC system results in a reduction in emissions from the reciprocating engine per unit of power generated. Moreover, the cascaded ORC system described herein reduces any waste heat from the first and second ORC systems. Thus, the method and system described herein results in improved efficiency of the reciprocating engine and each of the ORC systems.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A method of operating a cascaded organic Rankine cycle (ORC) system, the method comprising: vaporizing a first organic working fluid in a first ORC system using a high temperature heat source from a positive-displacement engine; heating a second organic working fluid in a second ORC system using a low temperature heat source from the positive-displacement engine; and vaporizing the second organic working fluid using heat from the first organic working fluid, wherein the first organic working fluid has a higher critical temperature than the second organic working fluid.
 2. The method of claim 1 wherein the positive-displacement engine is a reciprocating engine.
 3. The method of claim 1 wherein the high temperature heat source is exhaust gas, and the low temperature heat source is jacket cooling water.
 4. The method of claim 1 wherein a temperature of the high temperature heat source is between approximately 475 and 540 degrees Celsius, and a temperature of the low temperature heat source is between approximately 100 and 110 degrees Celsius.
 5. The method of claim 1 wherein vaporizing the second organic working fluid is performed by a heat exchanger configured to condense the first organic working fluid and to vaporize the second organic working fluid.
 6. The method of claim 1 wherein heating the second organic working fluid in the second ORC system is performed by a heat exchanger configured to extract heat from the low temperature heat source and to preheat the second organic working fluid.
 7. The method of claim 1 wherein the first organic working fluid is selected from a group consisting of siloxanes, toluene, isobutene, isopentane, n-pentane and 4-trifluoromethyl-1,1,1,3,5,5,5-heptafluoro-2-pentene ((CF₃)₂CHCF═CHCF₃).
 8. The method of claim 1 wherein the second organic working fluid is selected from a group consisting of R123, R134a, R236fa and R245fa.
 9. The method of claim 1 further comprising: heating an external source using heat from the second organic working fluid.
 10. (canceled)
 11. A waste heat recovery system comprising: a first organic Rankine cycle (ORC) system configured to vaporize a first organic working fluid using a high temperature waste heat source from a reciprocating engine, and to generate power using the first organic working fluid; a second organic Rankine cycle (ORC) system configured to receive heat from the first organic working fluid to vaporize a second organic working fluid, and to generate power using the second organic working fluid; and a heat exchanger configured to increase a temperature of the second organic working fluid using a low temperature waste heat source from the reciprocating engine and prior to vaporizing the second organic working fluid, wherein a critical temperature of the first organic working fluid is greater than a critical temperature of the second organic working fluid.
 12. The waste heat recovery system of claim 11 wherein the high temperature waste heat source passes through an evaporator of the first ORC system to vaporize the first organic working fluid.
 13. The waste heat recovery system of claim 12 wherein the first organic working fluid passes through a condenser located downstream of the evaporator of the first ORC system to condense the first organic working fluid and vaporize the second organic working fluid.
 14. The waste heat recovery system of claim 11 wherein the heat exchanger is located downstream of a condenser of the second ORC system and upstream of an evaporator of the second ORC system.
 15. The waste heat recovery system of claim 11 wherein the high temperature waste heat source is exhaust gas from the reciprocating engine, and the low temperature waste heat source is jacket cooling water from the reciprocating engine.
 16. (canceled)
 17. The waste heat recovery system of claim 11 further comprising: a heat sink configured to receive heat from the second organic working fluid and to provide heating to an external source.
 18. A method of operating a cascaded organic Rankine cycle (ORC) system having a first ORC system configured to circulate a first working fluid and a second ORC system configured to circulate a second working fluid, the method comprising: vaporizing the first working fluid in an evaporator of the first ORC system using exhaust gas from a reciprocating engine; heating the second working fluid upstream of an evaporator of the second ORC system using cooling water from the reciprocating engine; and vaporizing the second working fluid in the evaporator of the second ORC system using heat from the first working fluid of the first ORC system, wherein a critical temperature of the first working fluid is greater than a critical temperature of the second working fluid.
 19. The method of claim 18 wherein the evaporator of the second ORC system is configured as a condenser of the first ORC system.
 20. The method of claim 18 wherein the first working fluid is selected from a group consisting of siloxanes, toluene, isobutene, isopentane, n-pentane and 4-trifluoromethyl-1,1,1,3,5,5,5-heptafluoro-2-pentene ((CF₃)₂CHCF═CHCF₃), and the second working fluid is selected from a group consisting of R123, R134a, R236fa and R245fa.
 21. The method of claim 18 wherein a temperature of the exhaust gas exiting the reciprocating engine is between approximately 475 and 540 degrees Celsius, and a temperature of the cooling water exiting the reciprocating engine is between approximately 100 and 110 degrees Celsius.
 22. The method of claim 18 further comprising: heating an external source using heat from the second working fluid in the second ORC system. 